The present invention relates generally to optical fiber sensors for detecting, measuring, and monitoring dynamic events and changes in properties occurring within elastic material subject to strain and/or attrition or development. More particularly, the present invention is directed to optical fiber sensors formed of deformable elastomeric materials embedded within elastic materials such as aggregates including asphaltic and cementitious materials, elastomers, or within the body of an animal for detecting, measuring, and monitoring dynamic loads such as compression, stretching or bending loadings occurring within the elastic materials, and/or the attrition or deterioration of the elastic material such as caused by cracking, aging, aggregate break-up, moisture infiltration, disintegration of the material, or the development of the elastic materials such as caused by increased growth in animal muscles and organs.
This invention was made with the support of the United States Government under contract No. DE-AC05-84OR21400 awarded by the U.S. Department of Energy. The United States Government has certain rights in this invention.
Optical fibers are becoming of increasing interest in the development of sensors used for the detection and measurement of various events which apply strain to a material supposing the fiber optic sensor whereby attenuation of the light transmitted through the optical fiber is indicative of the amount of strain applied to the material. For example, optical fiber sensors have been developed in use for weighing vehicles in motion by arranging a selected array of fiber optic sensors along a roadway in the path of a moving vehicle. One such optical fiber sensor arrangement for weighing vehicles in motion is described in a commonly assigned and allowed U.S. Patent application entitled, "Apparatus For Weighing and Identifying Characteristics of a Moving Vehicle", J. D. Muhs et al, Ser. No. 07/864/888, filed Apr. 4, 1992. This commonly assigned patent application is incorporated herein by reference.
In other recent developments in which optical fibers are used in sensors designed to detect and quantify forces applied to the surface regions of various engineering structures such as bridges, aircraft components, vehicle bumpers, and the like is described in U.S. Pat. No. 4,734,577. In this patent, optical fibers formed of glass or plastic are attached to the surface of or encased in the structure that is to be monitored and are so mounted so as to have at least one flexible or curved section thereon, preferably a plurality of curved sections, so that any bending or straightening of the curved optical fibers due to surface motion in the structure or material being monitored will affect the radius of the curved section or sections and thereby change the amount of light being transmitted through the optical fiber. The amount of change in light transmission is indicative of the applied load. While such an optical fiber sensor arrangement provides satisfactory results when attached to the surface of the material to be monitored, some drawbacks or shortcomings are inherently present when attempting to embed the optical fiber in many materials. For example, during the curing of various structural materials such as asphalt or cementitiuous materials including concrete, mortar, and grout, and elastomers such as synthetic rubbers and various other polymers such as urethanes and polyethylene, various glass or plastic fiber-damaging forces such as caused by the expansion or shrinkage of the material will be imposed upon the glass or plastic fibers. Also, efforts to provide the glass or plastic optical fibers with the desired degree and number of bends would be difficult to achieve when attempting to embed the optical fibers in liquidous materials such as freshly poured concrete or pre-cured elastomeric materials. Furthermore, the sensitivity of such sensors in embedded arrangements is likely to be impractical since glass and plastic optical fibers are nondeformable and are not generally considered generally to possess the dynamic range and/or sensitivity for accurately sensing highly elastic or fracture prone materials.
Another development in optical fibers and optical fiber sensors which is of significant interest to the present invention is described in U.S. Pat. Nos. 4,830,461 and 4,937,029. For the purpose of this description these patents will be hereinafter referred to as the "Bridgestone" patents, the name of the assignee thereof. The optical fibers described in these patents are formed of a core and cladding of a relatively transparent elastomer such as a synthetic rubber selected from chloroprene rubber, urethane rubber, silicone rubber, acrylic rubber, fluorene rubber, ethylene-propylene rubber, ethylene-propylene diene, terpolymer rubber, and epichlorohydrin rubber. In these patents the pressure-sensitive sensors employing the optical fibers of the listed rubber elastomers are placed on a surface of a structure so that when a strain such as compression or tension or that of a bending force is imposed upon the structure, the optical fiber becomes deformed by changing the cross-section under a compression or tension loading or by bending the optical fiber. Such deformation of the optical fiber attenuates the light passing therethrough according to the amount of bending or strain applied to the optical fiber. While the pressure-sensitive optical fiber sensors in these Bridgestone patents provide several advantages over previously known pressure-sensitive sensors using optical fibers formed of glass or plastic, there are several significant areas in material behavior which have yet to be successfully monitored and measured by fiber optic sensors. For example, in the Bridgestone patents the pressure-sensitive sensors are not fixedly embedded in the structure being monitored and are thus incapable of undergoing virtually similar conjunctive displacement with the structure being monitored in response to dynamic loadings or a bending force being applied to the structure, especially at locations internal of the surface regions of the structure, and are thus of a sensitivity often less than desired for many applications. For example, such previously known pressure-sensitive sensors would be substantially incapable of detecting the formation of a crack in the structure supporting the optical fiber sensor, accurately measuring impact loadings such as caused by the foot of a human being impacting upon an elastic sole of a shoe, or strain on animal components such as muscles and organs. Also, with the previous pressure-sensitive sensors any attrition and/or development in the structure could not be accurately detected, let alone accurately measured. The detection of cracks and attrition in elastic materials such as cementitious materials and elastomers including synthetic rubbers and various plastics such as urethanes and polyethylene and the providing of highly accurate measurements of dynamic loadings, however slight, occurring in internal regions of the material being monitored is of considerable importance in determining the functional operation and the useful life of such elastic materials.
These Bridgestone patents at column 2, lines 23-52, point out further shortcomings of using optical fibers of grass or plastic in pressure-sensitive monitors such as described in aforementioned U.S. Pat. No. 4,734,517. Also, since the optical fibers described in Bridgestone patents with the core or cladding formed of the rubber elastomeric material are descriptive of the optical fibers used in the various embodiments of the present invention, these Bridgestone patents are specifically incorporated herein by reference.